1. Field of the Invention
The present invention relates generally to distributed constant type filter devices, and more particularly to a distributed constant type filter device applied to a flat panel antenna device using UWB (ultra-wide band).
2. Description of the Related Art
FIGS. 1A, 1B are schematic diagrams of a conventional ring filter device 10, which is a distributed constant type filter device. The ring filter device 10 includes a substrate 11 made of epoxy resin. A ring filter element 12 having an open stub is arranged on a top surface 11a of the substrate 11. A ground pattern 15 entirely covers a bottom surface 11b of the substrate 11.
The ring filter element 12 having the open stub includes a ring part 13 and an open stub part 14. The ring part 13 includes a first transmission line 13a having a length λ/2, and two second transmission lines 13b, 13c, each having a length λ/4. It is assumed that λ corresponds to a wavelength of a frequency f0. The impedance of the first transmission line 13a is Z1, the impedance of the second transmission lines 13b, 13c is Z2, and the impedance of the open stub part 14 is Z3.
The ring filter device 10 has a transmission property as shown in FIG. 2, with two attenuation pole frequencies f1, f2. A frequency band between the two attenuation pole frequencies f1, f2 is denoted by “A”.
The attenuation pole frequencies f1, f2 are determined by ratios between the impedance Z1 of the first transmission line 13a, the impedance Z2 of the second transmission lines 13b, 13c, and the impedance Z3 of the open stub part 14.
By decreasing the impedance Z3 of the open stub part 14, the frequency band A becomes wide; by increasing the impedance Z3, the frequency band A becomes narrow.
There are a variety of commercialized products with different frequency bands A that can be employed as the ring filter device 10. Thus, according to the product employed as the ring filter device 10, the impedance Z3 of the open stub part 14 has an appropriate value in the range of 10Ω through 100Ω. The ring filter device 10 is manufactured so that the open stub part 14 is designed to have predetermined impedance Z3.
Patent Document 1: Japanese Laid-Open Patent Application No. 2005-295316
In the conventional ring filter device 10, the impedances Z1, Z2, Z3 are determined by parameters such as a relative dielectric constant (∈r0) of epoxy resin used as the material for the substrate 11, the thickness of the substrate 11, etc.
The impedance Z3 is specifically described herein. For example, when the impedance Z3 is decreased to 10Ω in order to widen the frequency band A, the width W of the open stub part 14 is extremely wide, such as 20 mm. Conversely, when the impedance Z3 is increased to 100Ω in order to narrow the frequency band A, the width W of the open stub part 14 is extremely narrow, such as 0.1 mm.
Thus, in order to make the open stub part 14 have an appropriate width W, the impedance Z3 of the open stub part 14 is selected to be within a range narrower than 10Ω through 100Ω. This limits the freedom in the design of the ring filter device 10.